1. Field of the Invention
The present invention relates to a tandem-type vacuum booster and more particularly to a vacuum booster of the type wherein a partition plate is secured to a booster shell to divide the interior thereof into a front shell chamber and a rear shell chamber, the front shell chamber being divided into a front-side front vacuum chamber and a rear-side front operation chamber by a front booster piston and a front diaphragm superposed on the rear surface of the front booster piston, the rear shell chamber being divided into a front-side rear vacuum chamber and a rear-side rear operation chamber by a rear booster piston and a rear diaphragm superposed on the rear surface of the rear booster piston, the front booster piston and diaphragm and the rear booster piston and diaphragm being connected together via a piston boss which is slidably supported on the partition plate and leads to an output rod, and wherein a valve cylinder is formed continuously from the rear end of the piston boss and is slidably supported on the rear wall of the booster shell, and in the valve cylinder are disposed an input rod capable of moving forwards and rearwards and a control valve for alternately communicating both the front and rear operation chambers with the atmospheric air or the front and rear vacuum chambers.
2. Description of the Prior Art
A conventional tandem-type vacuum booster employs a piston boss which is divided into two parts in its axial direction, these divided parts serving to clamp and hold therebetween the rear booster piston and the rear diaphragm, the divided parts being connected together at their central portions by a screw member which is also used to fix the end wall plate of a connecting cylinder extending from the front booster piston onto an end surface of the piston boss. See Japanese Utility Model Application Laid Open No. 205859/1986, for example.
In a prior art structure, however, the piston boss is generally provided therein with a plurality of ports for communicating the operation chambers with the vacuum chambers and the atmosphere and if the piston boss is divided axially into two parts as suggested above, it then becomes necessary to interpose a seal member between the fitted surfaces of the divided parts so as to surround each communicating port at the time of coupling the two parts. This makes the structure complicated. Moreover, in the process of fixing the connecting cylinder of the front booster piston to the central portion of the piston boss by a screw member, the tightening torque of the screw member may cause rotation of the front booster piston, resulting in that the front diaphragm is undesirably twisted. Such twisting of the front diaphragm would undesirably deteriorate the sealing property and durability.